|dc.description.abstract||Cirrus clouds play a major role in the global radiation budget and consider as one of the uncertain components in the climate system. This calls for accurate parameterizations of cirrus clouds physical properties in different geographic locations, which are highly essential for climate modelling. The high temporal and spatial resolutions of lidar have proven to be valuable remote sensing tools to monitor the structure and dynamical processes of upper tropical cirrus clouds. At this juncture, by using a polarization diversity lidar (Nd:YAG, 532 nm), the cirrus clouds have been observed over Chung-Li (24.58 oN, 121.10 oE, 167 m MSL), Taiwan over a period from 1999 to 2009. This work synthesis the characteristics of cirrus clouds with a long term data base. It also focuses on the dynamical and the thermodynamical factors that control their formation and dissipation. Cirrus clouds were characterized in terms of the seasonal, geometrical, optical (viz., depolarization ratio, lidar ratio, extinction coefficient, and optical depth), and thermal properties. The occurrences of cirrus cloud are more in the summer and comparatively less in the winter. In order to characterize the temperature dependence of the optical and geometrical properties of cirrus clouds, we have utilized the temperature information from the radiosonde launched twice daily from the nearby lidar site (nearly 30 km).
A theoretical estimation of an effective size of ice crystals in cirrus clouds using fall velocity derived from the lidar measurements has also been attempted. The lidar derived mean effective size of cirrus crystals are also parameterized in terms of the cloud mid-height temperature as well as optical depth. It is found that the size of ice crystals becomes small with the decrease in temperature. This will reflect in the decrease in optical depth, which clearly revealed the manifestation of cloud-radiation interaction. An attempt has been made for the first time to understand the impact of planetary-scale disturbances on the formation and disappearance of cirrus cloud in the vicinity of tropopause (VOT). These wave disturbances greatly modulate the temperature in the VOT and during the cold phase of anomalies, the descending cirrus clouds are observed.
In addition, the role of redistribution of the tropical upper tropospheric humidity, which supports the formation of the tropical cirrus clouds, has been investigated by using the CALIPSO, Aura-MLS and NCEP/NCAR reanalysis data. Results show that the redistribution of upper tropospheric humidity from a highly convective zone to the Indian peninsular region leads to the formation of the tropical cirrus. Advection of upper layer humidity is caused by the tropical easterly jet (TEJ) associated with the Asian Summer Monsoon (ASM). Thus, the results bring out for the first time, the role of TEJ in redistribution of the upper tropospheric humidity and consequently, in the formation of tropical cirrus.